Tempering glass



June 9, 1942-. J. T. LITTLE-TON E-r Ax. 2,285,595 TEMPERING GLASS Y.

Fledsept. 12, 1939 /WVEALED INVENTORS. fesse' Zl/TTLEro/v HOWARDIRL/1.1.15.

Patented June 9, 1942 y -TEMPERJNG GLASS- Jesse Tfliittleton, Howard R..Lillie, and William W. Shaver, Corning, N. Y., Glass Works, Corning, N.

New York assignors to Corning Y., a corporation of Application September12, 1939, SerialNo. 294,540 (ci. 49-89) 8 Claims.

This invention relates'to tempering glass and more particularly to thecontrol of the degree of compression in the surface layers of a glassbody.

The compressive strength of glass is known to be much greater than itstensile strength and, consequently, in endeavoring to produce strongerglass, eiorts have been directed toward introducing compressionareasinto glass articles. As 4a result, tempering of glass was practiced4as early as 1874 by De La Bastie and others.

So far as we are aware: there has been no effort on thepart of priorexperimenters to govern the degree of compression in the surface layersof a glass article and prior art has wholly neglected the tension factorwhich mustv accompany and balance the zone of compression in everyarticle into which compression is introduced.

`I'he principal object of this invention is to produce a glass articlehaving permanently set within its b ody a modied condition of strainsuch1 that its surface layers are in compression and its center is intension, the ratio of compression to tension being relatively high.

A further object of the invention is a glass article having its surfacelayers in compression and its central layers in tension, a considerablenumber of the central layers in tension being substantially uniformlystressed.

YIn accomplishing this, considerationmust be.

given to the softening point, annealing point,- and "strain point of theparticular glass subjected to the tempering. l

softening point, as used herein, is that condition inwhich glass has aviscosity of 10V-6 poises; annealing point is that condition in whichglass has a viscosity of 1013-4' poises; and "strain point is thatcondition in which glass has a viscosity of 1014-6 poises. We also usethe expressions softening temperature, annealing temperature, and straintemperature as dening the temperature at which any annealed glassattains the viscous conditions above recited.- i The above and otherobjects may be accomplished `by so reducing .the viscosity of a glassarticle throughout its mass that it lies below 1013 4 poises andpreferably near 10"-6 poises, subjecting the article to a sudden shortstiffening treatment by which its surface layers only attain -rigidityarid then subjecting the article to a treatment by which its interiorportions attain its mass to a temperature lying between the annealingand softening temperatures of the glass 'from which the article is made,subjecting the article for a short period of time to a relatively severechilling medium held at a temperature low enough to cause the surfacelayers of glass to become set, but above that which would cause them tobreak or check, then immediately subjecting the article to a temperaturelying well above the lower temperature but below the strain temperatureof the glass from which it is made for a somewhat longer period of timeuntil all portions of the article achieve this temperature andsubsequently allowing the article to cool in air. A

The degree of temper obtained under specific chilling conditions iscontrolled primarily by the thermal expansion coefficient of the glassfrom which an article is made, though the thermal conductivity, thethickness of the glass, and its shape are also factors of somewhatlesser importance. The higher the expansion coeiiicient of the glass,the greater will be the degree of temper of the'article and the lowerthe expansion coeiiicient the less 'will be the degree of temper for alike treatment.

In the drawing:

Fig. 1 is a sectional view through a piece of annealed glass showingdiagrammatically the line of Zero stress on opposite sides of which anystress in the glass should appear;

Fig. `2 is a view similar to Fig. 1 showing diagrammatically thedistribution of stress on opposite sides of the line of zerovstress in apiece .4

of glass tempered by the methods heretofore employed;

Fig. 3 is a view similar to Figs. 1 and 2 illustrating diagrammaticallythe distribution of stress on opposite sides of the line of zero stressin a piece of glass which has been tempered in accordance with themethod of this invention by which a very high degree of compression canVbe introduced into the surface and a high ratio "square or rectangularstrain pattern produced. y

Theory teaches that the higher the degreeof 'compression' in the surfacelayers of anarticle -of glass, the higher will be its resistance tobreakage, and, therefore, for a given degree of maximum tensionl in theglass, the greater the ratio of compression to tension the stronger willbe the glass. Since the magnitude of the area A is a rigidity at'agreatly reduced rate. This may be measure of the compressive forces inone-half l of the glass. it must be equal to the area B which is 'ameasure of the tension forces in one-half the glass under tension (Fig.2), and the only way that a high ratio ofcompression to tension can beobtainedjsto lengthen the area A at the sacrifice of its depth. By thesame reasoning, to decrease maximum tension in an article, the tensionarea B must be shortened and broadened "so that the stress patternobtained will be generally of the character shown in Fig. 3 in which themaximum compressive stress is about ve times the maximum tensile str essthus producing a flve to one ratio, and the area `A representing againthe total compressive force is equal to the area B representing thetotal tensile force of one-half of the glass section.

It has been quite definitely determined that the final. strain pattern-set in a glass article isdetermined by the temperature gradientexisting in the various layers as they set up; i. e.,

pass thru that range of viscosity beyond which further molecularadjustment within the glass is impossible. Thus to achieve thestrainpattern here desired, it is essential that the article besubjected to an' extremely severe initial chill which will not onlyestablish an extreme temf perature gradient in the surface layers, butwill set up these layers against Afurther viscous flow and strainrelease. While -a severe initial chill is essential it must bediscontinued immediately upon setting up the' desired surface conditionsand replaced by a milder chilling action which is so controlled andproportioned to the thermal conductivity and capacity of the glass thata considerable portion of the central layers are set up under conditionsof comparatively low Aand generally decreasing temperature gradient.This 'second chill need be continued only so long' as is required to setup the entire article and bring it below its strain temperature afterwhich the article may be cooled to room temperature rapide ly or slowlyas desired.

In actual practice it has been found that the difliculty attending theestablishment of high `ratio patterns increases asthe thickness of theglass decreases. This is due :to the relatively low coefficient of,thermal conductivity of glass which decreases the effectiveness of thechilling medium as the thickness of glass through which it operatesincreases and results in a relatively hot central section in a thickersection and a relatively low temperature gradient therein on coolingeven tho the surface layers are subjected to a very severe chillingaction. Accordingly applicants illustrations of their inventionarelargely in terms of the more difcult thinner sections; altho typicaltreatments of thicker sections are also given. These cover a number ofrepresentative glasses of varying composition and physicalcharacteristics as follows: Y

Table I .e B y c D E F 73.0 50.5 54.5 7a 73 15.5 zas 22.5 1 2.5 5.o 11.5 4. a1 s o 15 15.5 5.5 5.5

MgO 3. 8o tening temp.. 703 696 Annealing temp. l550 496 510 Straintemp. 503 l 494 481 445 483 475 Coef. of expans.. 32 36 46 48 94 92Glass A above is shown as composition B1 of the Sullivan and TaylorPatent No. 1,304,623, while glass B is of the type shown in Kraus PatentNo. 1,508,455. Glasses'E and F are typi- A wide var1ety of proceduresmay be followed to produce square strain patterns. The glass ware may beheated in liquidv baths of suitable composition or in airso long as theentire article is raised to a temperature near to its softeningtemperature. The initial chill may be effected either by subjecting thearticle to a spray of finely divided liquid cooling medium or byimmersing the article momentarily in a liquid bath. Obviously the moresevere the chilling action of the, medium and the lower its temperaturethe shorter will be the time of initial chill. However, in order toproduce the desired strain pattern the initial chilling medium must besufiiciently severe to set up the surface layers .in a period of a fewseconds since otherwise the temperature gradient within these layers atthe time they set up will not be sufficiently high. As

`a general rule the maximum duration of the initial chill is roughlyproportional to the thickness ends for each one-fourth inch ofthickness.The initial chilling medium is generally used at a temperature at least100 C. below the strain temperature of the glass being processed andlower temperatures are often preferable. In general a 'spray action ismore severe than simple immersion in the same liquid at the sametemperature and accordingly shorter timesmay be employed. l

The second chilling of the article may advantageously' be performed in amolten salt bath since 'such baths are particularly adapted to maintainthe surface of the 'glass article at a uniform, relatively hightemperature while their high heat capacityA will aid in lowering thearticle to their temperature.l The nitrate and nitrite salts of sodiumand potassium are commonly used for this purpose. 11n general it hasbeen found desirable to maintain this bath temperature near, butsomewhat below, the strain temperature ofthe glass being processed, adifierence in the neighborhood o f 50?' C. usually being satisfactory.Thi-n ware will attain the temperature -ci' the bath in from twenty tothirty seconds, but since additional treatment at this temperature hasno appreciable' effect the second chill is usually maintained for aperiod of about one minute in ware of ten millimetervthickness or less.

While the maximum values of compression and terized by a relativelywidezone of substantiallyuniform stress in the center of the section..Articles stressed in this manner have a particularly high tensilestrength making this type of ware useful wherever high bending stressesor thermal shocks are to be'encountered.

The following Table II sets forth the results obtainedv in a numberl ofinstances 'in whichv glasses of the various compositions listed inTableI have been subjected to various treatments using different heating andcooling media and methods. In each instance a strain pattern of` thehigh ratio type was produced and the ratio of compression to tension wasgreater than 3.25.

e Same Table I! Glass composition A 'A A A A A Glass thickness 54" 0 mm10.5 mm..- 54" n" W. nesting temp 800 850 800 800 775.' Pre. 030. tFlash 850. Heatingtime 4min...` 3mir1 3min 5min 5min 5min. A l 2 min,Heating medium Air NaCl NaCl Air Air Air.

NarS0t.-... NarSO4 fp lst chill temp; 175. 250 160 160 90 145. lst chilltime 5sec. spray 3sec. spray.. 3sec. spray.. 2sec. spray.. 1 sec. dip.3sec. dip 4sec. dip

NaNOz. BNOI NBNOQ. NaNOs. 1st chlll medlum KN 0; KN Or KNO;.. KNO'z.

10% H10. 2nd chill temp 450 300 400 440 2ndchi1l time 2mm; 1min 1min..40500. 2nd chill medium NaNO; NnNO1 NaN0; NaNOs NaNOr KNOr- KNQa KN KNKNO KNO; Tension.. 1.1.. 1.5 2.2 2.2 Compression 10.3 0.2.-. 9.3 8.5 7.6Ratio c/'r 3.0 3.4

f Glasscomposition I B C Dl E F Glass thickness si" W 54" 54" W 1".

Pre. 400.... Pre. 400'. Heating temp... 750 700 700 750 Air Air 600.

. 10 min.. 15 mln. Heating time 3 min 3 min. 3 min. 1% min 7 min 10 min.

- 1 min 3 min Heating medium N aCl Kcl. NaCl NaCl NBrS04 NazS KzS04KzSOc y N8lSO1. NazSOl 1st chill temp 200 180 180 320 250 300. 1st chilltime 2 sec. spray.. l sec. 2% sec. 3 sec. 5 sec. 10 sec. dip... 20 sec.dip.

spray spray spray spray lstchill medium NHNOI Os. 2nd chill temp 450-2nd chill time 5 min 2nd chill medium NBNOI- KNOa.- Tension 2-0-Compression 7.9.

Ratio C/T 3.9.

-bath of higher temperature so that the cooling From the foregoing itwill be seen that reslow air cooling or quenching in hot water maygardless of the type of chilling medium employed, be used, asconvenient.

the necessary COditOnS fOr fOrmatiOIl 0f a high 60 This application isa. continuation-'impart of ratio strain pattern include an initialsevere chill our copending application serial No, 117,194,

for a period not exceeding five seconds per onemed December 22, 1936.

fourth inch'of glass thickness to set up the sur- We claim:

,face layers only of the glass followed by a (ool' 55 l. The method oftempering a. glass article mg down of the'remander of the artlcle m a'to produce a modied'distribution of permanent stress therein whichcomprises heating the en'- tire articlesto a. temperature approachingthe softening temperature of the glass from which rate is less severeand a substantially uniform temperature gradient exists in each of thecenal lafefs f the article as they Set up' After 70 the article is made,severely-chilling the article the article has reached the second bathtemperto set the surface layers only in rigid condition ature, whichobviously must be below the strain by means of a, clning medium having atemtlnpeliiure 0f the glass, the article may be perature substantiallybelow the strain temperachilled at any desired rate, without appreciablyture of the glass and immediately immersmg changing the strain patternset therein, so either 75 the article in a quenghmg bath maintained at atemperature higher than the temperature o! the initial chilling mediumbut below the strain temperature of the glass until the entire articleassumes this temperature and subsequently cooling the article to roomtemperature.

2. `The method of tempering a glass article to produce a modifieddistribution of permanent stress therein which comprises heating theentire article to a temperature approaching thesoftening temperature ofthe glass from which the article is made, severely chilling the articlefor a period not exceeding tive seconds per one-fourth inch of glassthickness by means oi a chilling medium having a temperaturesubstantially below the strain temperature of the glass to establish ahigh temperature gradient in the article and set the surface layers onlyin rigid condition and immediately setting u p the remainder of thearticle while maintaining a lower, substantially uniform temperaturegradient therein, by subjecting the article to a temperature lyingbetween the temperature of the initial chilling medium and the straintemperature of the glass and subsequently cooling the article to roomtemperature.

3. The method of tempering a glass article to produce a modifieddistribution of permanent stress therein winch includes heating thearticle to a temperature approaching the softening temperature of theglass from which the article is made, subjecting the article to a severechilling medium having a temperature substantially below the straintemperature of the glass for a short time to create a sharp temperaturegradient in the article and set the surface layers only rigidly .againstmolecular movement, and immediately subjecting the article to a secondless severe chilling medium for a time suflicient to lowerthe^temperature of the entire article below the strain temperature ofthe glass from which it is made, said second chillingmedium being heldat a temperature close to the strain temperature of the glass, andsubsequently cooling the article to 'room temperature.

4. The method of tempering a glass article to produce a mddiiieddistribution of permanent stress ctherein which comprises heating thearticle to a temperature approaching the softening temperature of theglass from which it is made, severely chillingl the article to set thesurface layers only in rigid condition by means of a chilling mediumhaving a temperature substantially below the strain temperature of theglass and immediately immersing the Aentire article in a quenching bathmaintained `at a temperature yabove the temperature of the initialchilling medium and approximately 50 C. below the strain temperature ofthe glass of the article for a time sucient to bring the entire body ofthe article to this temperature, and subsequently cooling the article toroom temperature.

5. The method of tempering glass articles to produce a modifieddistribution of permanent stress therein which includes heating thearticles to a temperature above the annealing temperature vof the glassfrom which the articles are made, suddenly cooling the articles bysubjecting them for a period of time not exceeding iive secer 'period oitime to a higher temperature substantially 50C. below the straintemperature of the glass from which the articles are made to setup theremainder of the articles and subsequently cooling the articles to roomtemperature.

6. The method of tempering .glass articles to produce a modifieddistribution oi permanent stress therein -which includes heating thearticles to a temperature above the annealing temperature of the glassfrom which the articles are made, suddenly cooling the articles bysubjecting them for a period of'time not exceeding five seconds yto achilling medium held at a temperature at least 300 C. below the straintemperature of the glass from which the articles are made to set thesurface layers only of said articles, and immediately subjecting thearticles for a somewhat longer period of time to a higher temperaturewhich is at least 50 C. below the strain temperature of the glass fromwhich the articles are made to set up the remainder of the-articles andsubsequently cooling the articles to room temperature.

7. The method of tempering a glass article to produce a modieddistribution of permanent stress therein which includes heating thearticle to a temperature above the annealing temperature of the glassfrom which it is made, suddenly cooling the article by subjecting it fora period of time not exceeding fl-ve seconds per one-fourthl to set upthe remainder of the articles and subsequently rcooling the articles toroom temperature.

' 8. The method of tempering a glass article to produce a modified..distribution of Vpermanent stress therein which includes heating it to atemperature above the annealing temperature oiEv the glass from which itis made, suddenly cooling the article by subjecting it for a period oftime -not exceeding iive seconds per one-fourth inch of glass thicknessto a chilling medium held at a temperature at least 200 C. vbelow thestrain temperature of the glass of the article to set up the surfacelayers only of said article and immediately subjecting it for a periodof at least one minute to a higher temperature which is approximately 50C. below the strain temperature of the glass from which the article ismade t0 set up the remainder of the articles and sub'- seque'ntlycooling the articles to room temperature.

JESSE 'I'. LITI'LETON. HOWARD R. LILLIE. WILLIAM W. SHAVER.

